When transmission of color TV signals were developed, a decision was made to have the broadcast equipment transmit one set of color signals that could be decoded by black and white TVs to show black and white images, and by color TVs to display color images. The resulting color encoding was called YUV format, and television broadcasters worldwide adopted this system. The signals in the YUV format are derived from the RGB format. The RGB color format is the format captured by the analog and digital cameras.
The Y channel in YUV is the “luma” or luminance channel, which carries brightness information, and is a sum of the R, G, and B color signals. The black and white TV sets decode only the Y part of the signal. The U and V channels in the YUV are the chroma channels of the YUV format, and carry the color information. Accordingly, the color TV sets will decode all three color components. The U channel carries blue minus luma (B-Y) information, and the V channel carries red minus luma (R-Y) information. Through a process called “color space conversion,” the video camera converts the RGB data captured by its sensors into either composite analog signals (YUV) or component versions (analog YPbPr or digital YCbCr). For rendering on a TV screen, these color spaces must be converted back again to RGB by the TV or display system. Various standardized equations are used for converting RGB to YUV.
The original TV standard combined the luma (Y) channel and both chroma channels (U and V) into one channel, which is known as “composite video.” An option known as “S-video” or “Y/C video” keeps the luma channel separate from the color channels, using one cable, but with separate wires internally. S-video provides a bit sharper images than composite video. When the luma channel and each of the color channels (B-Y and R-Y) are maintained separately, it is called “component video.” Component video is designated as YPbPr when in the analog domain and YCbCr when in the digital domain. In practice, YUV refers to the color difference encoding format whether composite or component, and “YUV,” “Y, B-Y, R-Y” and “YPbPr” are used interchangeably for analog signals. Sometimes, “YCbCr,” which is digital, is used interchangeably as well.
The primary advantages of the YUV system are that it remains compatible with black and white analog television. Another advantage of YUV system is that some of the information may be discarded in order to reduce bandwidth. The human eye has fairly low color sensitivity. That is, the accuracy of the brightness information of the luminance channel has far more impact on the image discerned than information in the other two chrominance channels. Understanding this human shortcoming, standards such as NTSC reduce the amount of data consumed by the chrominance channels considerably, leaving the eye to extrapolate much of the color information. NTSC saves only 11% of the original blue information and 30% of the red information. The green information is usually preserved in the Y channel. Therefore, the resulting U and V channels can be substantially compressed.
Because the human eye is less sensitive to color than intensity, the chroma components of an image need not be as well defined as the luma component, so many video systems sample the chroma channels at a lower sampling frequency than for the luma channel. This reduces the overall bandwidth of the video signal without much apparent loss of picture quality. The missing values will be interpolated or repeated from the preceding sample for that channel.
The subsampling in a video system is usually expressed as a three part ratio. The three terms of the ratio are: the number of brightness (“luminance” “luma” or Y) samples, followed by the number of samples of the two color (“chroma”) components: U/Cb then V/Cr, for each complete sample area. For quality comparison, only the ratio between those values is important, so 4:4:4 could easily be called 1:1:1. However, the value for brightness has been set to 4 traditionally, with the rest of the values scaled accordingly. There are other subsampling rates such as 4:2:2 and 4:2:0. Different video subsampling may be used due to different design specifications and/or implementations of different video standards. A problem may occur when a device receives video data with one subsampling rate while video data in that device is processed with another subsampling rate.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.